As for construction equipment such as a conventional hydraulic shovel, a hydraulically driven type, which controls an inflow of pressure oil discharged from a hydraulic pump with an engine as a power source into each hydraulic actuator so as to carry out drive of each kind of working machine, traveling of vehicle, and the like, is generally used. However, operations of a hydraulic shovel include not only the operations which always require 100% capacity of the capacity of the engine (maximum output horse power), but also a number of operations which can be performed with a smaller capacity (of 80%, for example) than them. Consequently, some hydraulic shovels perform a so-called equal horse power control in which the output of the engine is set in accordance with the level of the engine capacity required for the operation, namely, the magnitude of the working load, and the discharge amount of the hydraulic pump is controlled in accordance with a PQ curve (equal horse power curve). According to this control, the output power of the engine can be utilized effectively, and fuel consumption can be reduced.
The hydraulic shovel for performing the above-described equal horse power control has to be loaded with an engine having an output power corresponding to the maximum required horse power needed to perform the operation with the maximum load required of the hydraulic shovel. However, the load of the hydraulic shovel is not usually constant, and a load variation is very large due to repetition of the operations of excavation, rotation, traveling, earth-moving and the like. As a result, the average load rate with respect to the maximum horse power of the engine in one cycle of an operation is less than 100% (for example, about 80%) depending on the content of the operation, and accordingly, surplus occurs in the engine horse power. This results in the fact that the maximum capacity of the engine, namely, the power which the engine can output is not effectively used in the hydraulic shovel loaded with the engine having the output power equivalent to the maximum required horse power.
In order to solve the above problem, when the output torque of the engine is larger than the driving torque of the hydraulic pump, an inflow of pressure oil, which is discharged from the hydraulic pump, into a hydraulic actuator is controlled, while a first electric motor is driven as a generator with surplus torque of the engine and the generated electric power is charged into a battery. With this, an upper revolving superstructure is rotationally driven by a second electric motor receiving electric power from the battery. When the driving torque of the hydraulic pump becomes larger than the output torque of the engine, the first electric motor is rotationally driven as a motor by the electric power charged in the battery to assist the engine in driving the hydraulic pump. At the time of braking and stopping the upper revolving superstructure, the second electric motor is driven as a generator with the inertia energy thereof, and the generated electric power is charged into the battery. A hybrid construction equipment, in which at the time of lowering a boom, a hydraulic motor provided at a boom cylinder by bypassing is rotationally driven with return oil from the boom cylinder, so that the electric power generated by driving the generator with this rotation is charged into the battery, is proposed (for example, refer to Japanese Patent Laid-open No. 2002-275945).
According to the hybrid construction equipment as described above, when the working load is small, and the driving torque of the hydraulic pump is smaller than predetermined output torque of the engine, the first electric motor is driven as a generator with a surplus of the engine output power and the generated electric power is charged into the battery. On the other hand, when the working load is large, and the driving torque of the hydraulic pump becomes larger than the predetermined output torque of the engine, the first electric motor is rotationally driven as a motor by the electric power charged in the battery to assist the engine in driving the hydraulic pump. As a result, the load rate of the engine is made substantially constant whether the working load is light or heavy, and its average load rate is made large, thus making it possible to use engine output power effectively. The electric power generated by driving the second electric motor as a generator with the inertia energy of the upper revolving superstructure at the time of braking the rotation, and the electric power generated by the generator with potential energy by high-pressure return oil from the boom cylinder at the time of boom lowering are charged into the battery. As a result, the energy can be recovered and reused, and the substantial maximum required horse power of the engine can be made smaller than the prior art.
Incidentally, in the above-described hybrid construction equipment, all of the inertia energy at the time of braking the rotation of the upper revolving superstructure, which is recovered via the second electric motor, and the potential energy at the time of lowering the boom, which is recovered via the generator, as well as the surplus energy of the engine, which is recovered via the first generator, is converted into electric energy and charged into the battery. However, if all the energies are to be recovered reliably and charged into the battery, each of the electric motors, and the generator have to be increased in size, and there arises the problem of requiring a charging device such as a battery of large capacity, which is an increase in size of the devices of the charging system.